This invention relates to a ceramic powder compound from which there can be made a low-temperature sintered dielectric ceramic body of fine barium titanate grains and a small amount of cadmium silicate flux, the ceramic body having a high dielectric constant and a smooth temperature coefficient of dielectric constant, especially suitable of use in thin dielectric multilayer ceramic capacitors.
Dielectric ceramic compositions that, after sintering at about 1100.degree. C., lead to mature dielectric bodies with smooth standard X7R performance and high dielectric constant are described in the patents to Galeb Maher, U.S. Pat. No. 5,358,338 issued Nov. 2, 1993 and U.S. Pat. No. 5,010,443 issued Apr. 23, 1991, which are assigned to the same assignee as is the present invention
The first named of these patents employs a start powder having a simple composition that is essentially only a pure fine grain BaTiO.sub.3, a small amount of a cadmium silicate flux and a yet smaller amount of a sintering inhibitor compound, e.g. a niobium oxide. About 0.1 weight percent MnCO.sub.3 may be included. Multilayer bodies are formed of this powder and sintering is accomplished in a closed crucible at about 1100.degree. C. The thus sintered multilayer bodies have exceptionally desirable dielectric properties with one exception; the insulation resistance at a high life test temperature of 125.degree. C. and at a high voltage (about three times rated voltage, namely 150 volts) tends to drop below the limit of 100 megohm-microfarads.
It is shown in the patent that this problem is essentially solved by performing an additional step of annealing for an hour or two in an open atmosphere at a temperature a little lower than that at which sintering had been done.
A ceramic powder composition conforming to the teaching of the above noted patent U.S. Pat. No. 5,258,338 is designated herein as the composition of Example 1, corresponding to Table I. It has now been used in commercial quantities for several years by a number of manufacturers of multilayer ceramic capacitors.
In the manufacture of multilayer ceramic capacitors, this commercial powder is typically formed into an almost dry ceramic paste layer or tape that includes an organic binder. A thin film of 30% palladium and 70% silver electrodes are then screen printed on multiple pieces of the green ceramic tape which tape pieces are stacked to form a green ceramic stack structure or chip. After baking out the binder at about 400.degree. C., the chips are then bisked to harden the chips by heating to about 800.degree. C.
The chips are subsequently fired, i.e. sintered at from 1100.degree. C. to 1120.degree. C. in a closed crucible to cure the ceramic to a high density and to imbue the ceramic body with a high dielectric constant.
Sintering in a closed crucible is necessary for creating an atmosphere ambient to the chips that contains cadmium oxide vapor which is driven out at the sintering temperature from the cadmium silicate sintering flux in the chip bodies. Cadmium silicate compounds are not glass compositions and do not melt at the sintering temperature, as was pointed out by G. Maher in the patent U.S. Pat. No. 4,266,265. It is explained in the patent that the high temperature melting cadmium silicate flux reacts with other elements in the start powder mixture at the low sintering temperature of about 1100.degree. C. whereby the composition of the flux is altered and the flux is thus rendered liquid at the sintering temperature. In the composition of the patent U.S. Pat. No. 4,266,265, the molten flux serves during sintering as a reservoir through which a further exchange of cations in the ceramic body and cations in the flux it self my take place.
But the start materials of the flux in U.S. Pat. No. 4,266,265 included lead which reacts at sintering with the flux and drops the melting temperature.
Other such elements that may render the flux molten are the low temperature glass formers boron and bismuth, and they are not either in the present invention. Therefore it is believed that, in the capacitors of the patent U.S. Pat. No. 5,258,338 and those of the present invention, there were no such elements to react with the cadmium silicate and cause it to melt, and the reactions during sintering between the elements of the start materials at sintering is effected only by solid state diffusion aided by the atmosphere of the very chemically active cadmium vapor in the closed crucible.
Thus in the start powders of this patent and in the present invention, no such elements are provided which in combination with the cadmium flux cause it to melt during sintering.
An additional reason for sintering in a closed crucible is that the escape of large amounts of cadmium oxide during firing would otherwise create a serious health hazard to those involved in manufacturing capacitors.
The above-noted anneal step was thus also seen as a necessity but has been found by further experience to be burdensome to capacitor manufacturers for several reasons. The post-sinter anneal step is an additional manufacturing step entails additional cost and is a last step in which lack of sufficient process control can ruin the nearly finished capacitors. It also results in an increase of from five to ten percent in the dielectric constant. It also unfortunately causes the dissipation factor (DF) to become a little higher and the temperature coefficient a little less smooth.
It is therefore an object of this invention to provide a low-firing dielectric powder composition including a cadmium silicate flux that may be sintered in a closed crucible without a subsequent anneal step, leading to a mature ceramic having a smooth X7R temperature characteristic, a high dielectric constant and having at high operating temperatures a high insulation resistance.